Microstructural characteristics of PEO coating: Effect of surface nanocrystallization

The microstructural characteristics of plasma electrolytic oxidation (PEO) coating and the formation mechanism of the barrier layer in the coating were investigated. Transmission electron microscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscopy-high-angle annular dark-field imaging were used to investigate the morphology, composition, and microstructure of PEO coating and ultrasonic surface rolling process (USRP)-PEO combined coating. The results illustrated that surface nanocrystallization of the USRP-treated titanium alloy promoted the formation of the barrier layer, but had no obvious influence on the porous outer layer of PEO coating. The barrier layer formed on the surface nanosized titanium alloy was thicker (by ∼27%) than that formed on the original substrate, confirming that this layer was formed via O^2- and Ti⁴⁺ inter- migration and diffusion. Moreover, the dynamic growth process of this layer could be described as a cyclic process of “formation via ionic migration and diffusion→ dielectric breakdown”. The oxide coating was composed of nanocrystalline and amorphous phases resulting from the different cooling rates at different depths in the coating and heterogeneous distribution of phase compositions in the coating. In addition, the reason for the inhomogeneous distribution of P and Si (from the electrolyte) in the coating was clarified.